Adults with post-traumatic stress disorder (PTSD) who were abused or mistreated as children have dramatic differences in gene expression from those who suffered trauma later in life, according to a new study. The findings may reveal a biologically distinct subtype of the psychiatric disorder.

A growing body of evidence suggests that symptoms of PTSD involve distinct epigenetic alterations, which change how segments of a person's genome are expressed without altering the DNA itself. Looking for such biological markers can help diagnose and identify risk for PTSD.

The new research, published this week in the journal Proceedings of the National Academy of Sciences (PNAS), adds strong findings in support of the epigenetic theory.

"These are some of the most robust findings to date showing that different biological pathways may describe different subtypes of a psychiatric disorder, which appear similar at the level of symptoms but may be very different at the level of underlying biology," said Emory University School of Medicine researcher Dr. Kerry Ressler in a news release.

The research team, led by Dr. Divya Mehta and Dr. Elizabeth Binder of the Max-Planck Institute of Psychiatry in Munich, Germany, examined blood samples from 169 individuals who participated in the Grady Trauma Project, a study of thousands of Atlanta civilians with high risk for PTSD after exposure to violence, physical trauma, and sexual abuse throughout their lifetimes.

The participants, mostly middle-aged African-Americans, were separated into three groups: 108 people who had experienced lifetime trauma but did not develop PTSD, 32 who developed PTSD after childhood abuse, and 29 who developed PTSD without childhood abuse.

Mehta's team then analyzed the blood samples to look for patterns in a genetic modification called DNA methylation, which can indicate which stress-related genes are turned "on" or "off."

The results showed that, while the PTSD patients all experienced similar symptoms like nightmares, flashbacks, hypervigilance, and avoidance of trauma triggers, the epigenetic evidence showed strong differences between the genetic expression patterns of those with and without childhood abuse.

PTSD patients with past childhood abuse had more alterations in genes related to brain development and immune regulation, and had a higher rate of DNA methylation changes.

Those whose PTSD came from later trauma, on the other hand, had more changes in genes that regulated cell growth and promoted cell death.

Both patterns suggest that PTSD develops as a result of epigenetic changes at different points in life, though the different pathways likely involve different biological mechanisms that lead to similar psychological symptoms.

Whatever those mechanisms are, the study indicates robust support for the idea that changes in gene expression can severely alter a person's stress resilience and risk for PTSD later in life.

"Traumatic events that happen in childhood are embedded in the cells for a long time," Binder said in the news release. "Not only the disease itself, but the individual's life experience is important in the biology of PTSD, and this should be to be reflected in the way we treat these disorders."

Further research on these distinct biological pathways may lead to therapies specifically targeted to PTSD involving the presence or absence of previous child abuse, and indicate whether people who suffered childhood abuse may be at risk of developing PTSD from exposure to other traumatic situations like military combat.